mkdocs-material/material/partials/toc.html

<nav class="md-nav md-nav--secondary">
  {% set toc = page.toc %}
  {% if "\x3ch1 id=" in page.content %}
    {% set toc = (toc | first).children %}
  {% endif %}
  {% if toc and (toc | first) %}
    <label class="md-nav__title" for="toc">Table of contents</label>
    <ul class="md-nav__list">
      {% for toc_item in toc %}
        {% include "partials/toc-item.html" %}
      {% endfor %}
    </ul>
  {% endif %}
</nav>